This invention relates generally to fluxers for applying a liquid flux to printed circuit boards, and more particularly to a flux control device which is capable of maintaining the specific gravity and level of a liquid flux contained within a vessel in predetermined ranges.
Numerous techniques, both automatic and manual, have thus far been practiced for successively feeding printed circuit boards to a soldering zone to receive a dip soldering treatment therein. In such a case, it is the general practice to convey the printed circuit boards to a fluxing zone for applying a liquid flux to the soldering portions prior to the dip soldering in order to obtain good soldering. As for the liquid flux, an undiluted or concentrated flux containing a resin as its major component is usually used after dilution into a predetermined concentration range with an alcohol or other suitable solvents. The liquid flux is contained in a vessel and applied to the lower side of a printed wiring board by an overflow, spray or other method. During the fluxing operation, it is desired to maintain the specific gravity and level of the liquid flux within the vessel in predetermined ranges since otherwise variations occur in its specific gravity and the level due to evaporation of the solvent and consumption of the flux.
FIG. 1 illustrates a conventional flux control device, in which designated by the reference numeral 1 is a vessel for containing a liquid flux 2 to be applied to printed circuit boards, not shown. The vessel 1 is provided with a specific gravity detector 3, a temperature detector 4 and a liquid level detector 5 for measuring the specific gravity, temperature and surface level, respectively, of the liquid flux 2.
The level detector 5 has three electrodes 5a, 5b and 5c with their tips positioned at different vertical levels, detecting whether or not the surface level of the liquid flux transcends predetermined upper and lower limits L.sub.u and L.sub.l by electric conduction or non-conduction across electrodes 5a and 5c or across electrodes 5b and 5c.
Indicated at 7a and 7b are solenoid valves, at 8 a pump, at 9 a reservoir for a concentrated flux 10, at 11 a reservoir for a diluent liquid 12, and at 13 a feed pipe means. A control unit 6 receives output signals from the specific gravity detector 3, temperature detector 4 and liquid level detector 5, monitoring whether or not the liquid flux is between predetermined upper and lower limits .rho..sub.u and .rho..sub.l of specific gravity and upper and lower limits L.sub.u and L.sub.l of liquid level, and, if not, producing instruction signals for actuating the solenoid valves 7a and 7b. More specifically, the input value of the specific gravity detector 3 is corrected to a value of a certain reference temperature on the basis of the input value from the temperature detector 4, and the corrected value is compared with the values of the preset upper and lower limits .rho..sub.u and .rho..sub.l. When it is higher than the upper limit .rho..sub.u, the control unit 6 produces an output for actuating the solenoid valve 7b and pump 8 to feed the diluent liquid 12 to the vessel 1. On the contrary, when it is lower than the lower limit .rho..sub.l, the solenoid valve 7a and pump 8 are operated by the output of the control unit 6 to feed the concentrated flux 10 to the vessel 1.
Meanwhile, when the surface level of the liquid flux 2 in the vessel 1 reaches its predetermined lower limit, the level detector 5 produces an output of a lower limit value L.sub.l to feed the concentrated flux 10 or the diluent liquid 12 to the flux vessel 1 by alternately actuating the valves 7a and 7b along with the pump 8 until the liquid level reaches its predetermined upper limit while controlling the specific gravity within the predetermined range by the control unit 6.
However, the above-mentioned conventional device has a drawback in that due to the slow stop action of the pump 8 an instantaneous cut-off of the concentrated flux 10 or diluent liquid 12 is difficult even if the solenoid valves 7a and 7b are operated with a quick response. In addition, cleaning of the feed pipe of the undiluted flux is required every day at the end of operation since it is difficult to remove solidified flux from the pump 8.